Effect of time of feeding carbohydrate supplements and pasture on production of dairy cows

Abstract

The productivity and rumen status of Friesian cows grazing short rotation Italian ryegrass ( Lolium multiflorum) pastures was examined in relation to the time of grazing and of feeding cereal-based concentrates. In experiment 1, 42 cows grazed as seven separate groups of six cows on ryegrass pasture over a 2-day period, and were fed 2 kg crushed barley grain/cow at morning and evening milking on the first day. On the second day, no concentrates were fed at morning milking and a different group of cows was removed at random from grazing and a rumen sample collected by stomach tube at 0, 1, 2, 3, 4, 5, 7 and 9 h after commencement of grazing at 07:00 h. Sampling concluded before afternoon milking at 17:00 h. Rumen ammonia (NH 3) concentration peaked and pH fell to its lowest value between 7 and 9 h after cows commenced grazing. In experiment 2, 42 cows were stratified into three groups of 14 cows on the basis of milk and component yield, age, liveweight and calving date and randomly allocated within blocks to one of three treatments. Over a 10-day adjustment period and a 21-day experimental period, cows were given their daily pasture allocation of 13.5 kg DM/cow (above 5 cm stubble height) either after morning milking, and fed crushed barley grain at morning and evening milking in the ratio of 3:1 kg/cow (Synch) or 1:3 kg (Asynch) or given their daily allocation of pasture after afternoon milking with barley fed at a ratio of 3:1 kg/cow (PM) in the morning and evening milking. Based on the results of experiment 1, the feeding schedule for the Synch group was predicted to more closely synchronise the availability of N from pasture and readily fermentable carbohydrates (RFC) from concentrates, in the rumen, compared to the Asynch cows. The comparison between Synch and PM groups was to determine the relative importance of ‘synchrony’ compared to feeding pasture (afternoon) high in water soluble carbohydrates (WSC) content. The WSC content of ryegrass pasture sampled after morning milking (when the Asynch and Synch groups received their new pasture allocation) was 70 and 74 g/kg DM, respectively, compared to 124 g/kg DM for samples taken after afternoon milking (when the PM group received a new pasture allocation). Pasture dry matter intake (DMI) and in vivo digestibility of individual cows was determined by use of alkanes as inert markers, whilst group DMI was determined from pre- and post- grazing pasture mass estimated by rising plate meter. Rumen samples were taken by stomach tube at 0, 3, 7 and 11 h after grazing commenced in the morning on the last day of the experiment. On this same day the % of cows grazing was recorded at 10-min intervals. Nitrogen intake (733 g N/cow/day) was 280% above requirements and this resulted in rumen NH 3 concentrations being three times higher than the concentration at which microbial protein synthesis (MPS) would be expected to become limited. Milk (25.1, 24.3 and 26.8 l/cow/day; ( P=0.05)) and protein (809, 794 and 850 g/cow/day; P<0.05) yield for the Asynch, Synch and PM groups, respectively, differed significantly. Pasture DMI was not significantly different ( P>0.05) but DM digestibility was ( P<0.001) at 84% for the mean of the Asynch and Synch cows compared to 88% for the PM cows, respectively. The results of the present study indicate a substantial benefit of feeding cows pasture with a high WSC content. This can be achieved by giving the cows their daily allocation of pasture after afternoon milking in the knowledge that WSC accumulates during the day and that over 70% of the intake of pasture by cows will be consumed in the first 3–4.5 h of grazing a fresh pasture allocation. Under the conditions of this study there was no production benefit from attempting to synchronise the availability of N and RFC in the rumen by timing of WSC supplementation. This presumably indicates that MPS was not limiting the daily milk yield of 25–26 l/cow when these cows consumed high N ryegrass pastures.